There are numerous instances where it is desired to distribute over wire within a relatively local area, such as a building, wideband signals modulated on to RF carriers. A particular application is the distribution of video signals. For example, a school may have a number of classrooms and administrative offices, each having a television monitor, and it may be desired at a given time to provide a program to all of the classrooms and offices, originating either from a source within one of the classrooms or offices, such as a VCR or from an outside source, such as a local cable system. Similarly, a corporation may have a building, or several closely spaced buildings, with numerous conference rooms equipped with television monitors and analogous program presentations may be desired. It is desirable in such a distribution system to have infrastructure which is relatively inexpensive and provides for easy expandability.
Since new construction is commonly prewired by the local telephone company using category 5 twisted pair cable, a system has been developed to distribute video or other wideband signals through this existing infrastructure. Such a system is disclosed in related patent applications Ser. Nos. 08/548,038 and 08/548,036 filed Oct. 25, 1995 respectively by the assignee hereof.
As shown in FIG. 1, the system consists of a distribution unit or a series of distribution units which may be cascaded to each other in a main wiring closet. These distribution units are connected via the category 5 twisted pair infrastructure to a plurality of outlets in different rooms. Each outlet may either be used as an input or an output. When used as an input, a baseband source signal such as one coming from a VCR is modulated and injected into the system through a breakout box having an enabling circuit which activates the input line. When used as an output, a monitor may be simply plugged into any wall outlet and tuned into the desired channel in order to receive the distributed program. This system is designed to support a plurality of channels in the frequency band of 7 Mhz to 211.25 Mhz.
The main component of this system is the distribution unit. FIG. 2 shows a block diagram of the distribution unit which includes the signal inlet, a signal outlet and switch means operable to either first state or a second state. When the switch means is in its first state, it couples the transmission path to the combiner and the splitter. When the switch means is in its second state, it couples the combiner to the signal outlet instead of to the transmission path and couples the splitter to the signal inlet instead of to the transmission path. This system may include several distribution units cascaded from each other as shown in FIG. 1 whereby one of the outlet ports of the first distribution unit is connected to the signal inlet of the second distribution unit and one of the input ports of the first distribution unit is connected to the signal outlet of the second distribution unit. The switch means of the first distribution unit is in its first state and the switch means of each of the cascaded distribution units are in a second state. The first distribution unit is therefore considered to be operating in the master mode and the subsequent cascaded distribution units are considered to be operating in the slave mode.
A problem exists in this system, however, in that unwanted signals result from crosstalk between the outbound signal path and inbound signal path at the connectors and other conductive and radiated sources within the distribution unit. The problem is increased due to the fact that the unwanted signals are coherently applied to the output connectors resulting in an in phase undesired signal appearing at all input connectors. The undesired signal is increased by combining eight separate undesired in phase signals at the combiner input.